The present disclosure relates to a reading module that radiates light to an illumination object and reads reflected light reflected thereon and to an image forming apparatus provided with the same.
Conventionally, as a reading method of an image reading device incorporated in a multi-functional peripheral or the like using an electrophotographic process, there are known a CCD method using a charge coupled device referred to as a CCD (charge coupled device) sensor and a CIS method using an optoelectronic conversion device referred to as a CMOS (complementary MOS) sensor.
In the CCD method, by use of a plurality of plane mirrors and optical lenses, a reduced image is formed on an image sensor whose size is ⅕ to 1/9 of a size of an original document and then is read. One advantage of the CCD method is that a deep depth of field is obtained. Herein, a depth of field refers to a range in which, even when a subject (herein, an original document) is displaced in an optical axis direction from an accurately in-focus position, the subject is seen as if the subject is in focus. This means that, with a deep depth of field, even when an original document is displaced from a predetermined position, a fairly satisfactory image can be read.
On the other hand, one disadvantage of the CCD method is that an optical path length (a distance light travels from a subject to a sensor) is as extremely long as 200 mm to 500 mm. In an image reading device, in order to ensure such an optical path length within a limited space in a carriage, a plurality of plane mirrors are used to cause a light travel direction to vary. This undesirably leads to an increase in component count and thus to a cost increase. Furthermore, in a case where lenses are used in an optical system, chromatic aberration occurs due to a difference in refractive index depending on a wavelength. Correcting chromatic aberration requires the use of a plurality of lenses. Such use of a plurality of lenses also results in a cost increase.
In the CIS method, by use of a plurality of erect equal-magnification rod lenses arranged in an array, an image is formed on an image sensor that is equal in size to an original document and then is read. One advantage of the CIS method is that, compared with the CCD method, a relatively short optical path length of 10 mm to 20 mm is obtained, and thus miniaturization is achieved. It is also advantageous that imaging is performed using the rod lenses alone without requiring the use of mirrors, which are required in the CCD method, so that a scanner unit incorporating a CIS sensor therein can be reduced in thickness and has a simple structure, thus achieving a cost reduction. The CIS method is disadvantageous, however, in that a depth of field obtained is extremely small, and thus when an original document is displaced in an optical axis direction from a predetermined position, a significant influence of blurring results from image bleeding due to deviations in magnification between the individual lenses. As a result, a book document or an original document with an uneven surface cannot be read uniformly.
In recent years, there has been proposed a method in which, unlike in the above-described CCD method and CIS method, an image is read using a reflection mirror array in an imaging optical system. In this method, by use of a plurality of reflection mirrors arranged in an array, an original document read on a region-by-region basis in different reading regions corresponding to the reflection mirrors, respectively, is formed into a reduced-scale inverted image on a sensor. In this method, however, unlike in the CIS method using the rod lens array, one region is read and formed into an image by use of one optical system. Furthermore, since an imaging method using a telecentric optical system is adopted, when an original document is read on a region-by-region basis in a plurality of regions, there occurs no image bleeding due to images with different magnifications overlapping with each other, thus suppressing image blurring, so that a compound-eye reading method is established.
Moreover, in this method, the optical system uses the mirrors alone, and thus unlike in the case where lenses are used in an optical system, there occurs no chromatic aberration. Consequently, there is no need for correction related to chromatic aberration, and thus the number of elements constituting the optical system can be reduced.